1. Field of the Invention
This invention relates to novel polysaccharides and, more particularly, to water-soluble polysaccharides capable of use in various applications, including oil field applications such as hydraulic fracturing fluids, gravel packing, and the like.
2. Description of the Prior Art
Hydraulic fracturing fluids are widely used to stimulate production from oil and gas wells completed in low permeability formations. The fracturing fluid is pumped at a rate sufficient to open a fracture in the exposed formation and to extend the fracture from the well bore into the formation. Continued pumping of the fracturing fluid containing a proppant such as sand into the fracture results in proppant placement within the fractured zone. Following this treatment, the resulting fracturing fluid is recovered from the well, leaving the proppant remaining in the fracture, thereby preventing the complete closure and forming a permeable channel extending from the well bore into the formation.
The conductivity of the propped fracture depends in part on the size of the propped particles placed in the fracture. Such conductivity, in turn, depends upon the width to which the particular fracture may be opened during injection of the fracturing fluid. This will normally require that the hydraulic fracturing fluid employed have a relatively high viscosity.
The use of a high viscosity fracturing fluid is advantageous. Such fluids can support propped particles suspended therein without excessive settling. Also, relatively large propped particles can be placed in the formation using high viscosity fracturing fluids since wider fractures generally result, reducing the possibility of proppant particles bridging across the mouth of the fracture and accumulating in the well bore, a condition sometimes referred to as "screen-out".
However, the use of high viscosity fracturing fluids is disadvantageous due to excessive friction losses often encountered during the injection of such fluids into the formation through the tubing or casing disposed in the well bore. Thus, the pumping equipment and tubular goods available at the wellhead are often limited, and the wellhead pressure and the hydraulic horsepower required to overcome such friction losses are often prohibitive.
In addition, the fracturing fluids are exposed to high shear conditions while being pumped down the well bore. Suitable fracturing fluids must accordingly be capable of undergoing such high shear without involving any undue loss in stability, to minimize or eliminate problems, such as screen-out.
Still further, as the fracturing fluid extends out of the well bore into the fracture, the temperatures encountered substantially increase. It is accordingly necessary for fracturing fluids to be useful at such elevated temperatures, desirably up to 200.degree. to 350.degree. F. or more. While techniques are known to decrease the high temperatures involved (e.g., initially using sacrificial fluid to cool the formation) or the effects thereof (e.g., the use of auxiliary thermal stabilizers), the hydraulic fracturing fluid still must adequately perform at elevated temperatures. Thus, undue thermal instability at such elevated temperatures will cause problems such as fluid loss into the formation as well as the inability to carry sand into the formation, both of which translate into decreased productivity.
Lastly, after the proppant is in place in the fracture, the fracturing fluid must be capable of being readily removed so that oil or gas production can commence. In this regard, the fracturing fluid must be capable of being removed without any significant amount of residue remaining in the fractured formation which would decrease the permeability thereof, thereby decreasing productivity.
These diverse and demanding criteria have engendered substantial effort to provide a satisfactory fracturing fluid. Over the years numerous solutions have been proposed.
One general approach which has resulted in extensive efforts involves the use of a fracturing fluid containing a polymer and a cross-linking agent for the polymer. In theory, such a fracturing fluid can provide viscosity adequate to carry the proppant down the tubing into the fracture and will then provide a very high viscosity due to the cross-linked system that is formed in situ. The fracturing fluid may then be removed by either the action of a delayed action breaking agent which is included in the fracturing fluid or by the elevated temperatures which the hydraulic fluid encounters in the fractured formation.
U.S. Pat. No. 3,058,909 (Kern) discloses aqueous fluids having improved solids-suspending properties and improved fluid-loss characteristics for use in treating subsurface earth formations which incorporate in an aqueous medium a complex of a polyorganic compound having at least one reactive unit consisting of two adjacent hydroxyl groups arranged in a cis configuration and a boron compound capable of supplying borate ions in the aqueous solution. High molecular weight carbohydrates are most desirable, particular materials disclosed including guar and locust bean gum. The necessity of employing polyorganic compounds having at least one reactive group consisting of two adjacent hydroxyl groups arranged in a cis configuration to form the complex is illustrated by comparing materials of this nature with other carbohydrates whose chemical structure are quite similar except for the absence of the cis configuration.
U.S. Pat. No. 3,163,219 (Wyant et al.) sets forth a self-breaking, borate-gum, water-base gel utilized in oil, gas and water wells. Such gels are prepared by reacting an organic compound having at least one reactive unit consisting of two adjacent hydroxyl groups arranged in a cis configuration with a boron compound capable of supplying borate ions in an aqueous solution. The composition also includes a delayed action, gel-breaker such as solid calcium sulfate and the like.
U.S. Pat. No. 3,215,634 (Walker) relates to a method for reducing the temperature sensitivity of polymer solutions containing borate ion, cross-linking agents. The use of a polyhydric alcohol and a crosslinking agent which yields borate ions in solution stabilizes such solutions and reduces their sensitivity to temperature. The water-soluble polymers utilized are high molecular weight polysaccharides having adjacent cis hydroxyl groups attached to one or more of the monosaccharide units in the polymer structure. Preferred polysaccharides include the galactomannans.
U.S. Pat. No. 3,696,035 (Nimerick) discloses fracturing compositions which include a periodic acid and/or a water soluble salt thereof in an aqueous-alcohol mixture which has been thickened with a cellulose derivative. The thickened mixture reverses to a relatively flowable fluid after a period of time. Cellulose derivatives having the degree of substitution (D.S.) providing solubility in aqueous or aqueous-alcohol mixtures and which provide a viscous system having a minimum viscosity of about 10 centipoises when 0.25% by weight of the cellulose is dissolved in an aqueous or aqueous-alcohol mixture at a temperature of 80.degree. F. are suitable. Numerous cellulose derivatives which may be employed as a thickening agent are described, including cellulose ether, ethyl hydroxyethylcellulose, ethyl methylcellulose, hydroxyethyl cellulose and the like.
U.S. Pat. No. 3,727,688 (Clampitt) relates to a fracturing fluid comprising an aqueous gel, the gel including a water-thickening amount of a water-soluble cellulose ether and a water-soluble compound of a polyvalent metal wherein the metal is capable of being reduced to a lower polyvalent valence state and which is sufficient to gel the water when the valence of at least a portion of the metal is reduced to such lower valence state. Suitable cellulose ethers include carboxyalkylcellulose, mixed ethers such as carboxyalkyl hydroxyalkyl ethers, hydroxyalkylcellulose and the like.
U.S. Pat. No. 3,741,894 (Storfer) states that certain long chain water-soluble or water-dispersible organic polymers that are not normally susceptible of cross-linking with trivalent chromium and similar polyvalent cations can be chemically modified to permit such cross-linking to form modified polymers useful in the preparation of oil field drilling fluids and similar compositions. Thus, water-dispersible organic polymers having hydroxyl groups located in .beta. positions with respect to one another can be reacted in an aqueous solution with .alpha.-keto carboxylic acids containing from about 2 to about 6 carbon atoms per molecule to form cyclic ketals. The reaction products possess the essential characteristics of the unmodified polymers but undergo cross-linking reactions with trivalent chromium and other polyvalent cations in an aqueous solution, apparently through an olation mechanism.
U.S. Pat. Nos. 3,768,566 and 3,898,165 (Ely et al.) disclose a method for increasing the viscosity of the fluid at a time when the fluid is being subjected to temperatures which tend to reduce the initial viscosity. The viscosity is increased by the hydration of an additive which is a polysaccharide that has been cross-linked such that the hydration rate of the polysaccharide is greatly retarded at temperatures below about 100.degree. F. However, the bonds between the cross-linking agent and the polysaccharide are temperature sensitive, breaking at temperatures above about 140.degree. F., thereby enabling the aqueous fluid to hydrate the polysaccharide. Useful cross-linking agents are certain dialdehydes. Useful polysaccharides are those having a molecular weight of at least about 100,000 including galactomannan gums and cellulose derivatives including carboxymethylcellulose, carboxymethylhydroxyethylcellulose, and hydroxyethyl cellulose. The preferred gelling agent is hydroxyethylcellulose having an ethylene oxide substitution within the range of about 1 to about 10 moles of ethylene oxide per anhydroglucose unit.
U.S. Pat. No. 3,848,673 (Clampitt et al.) and U.S. Pat. No. 3,978,928 (Clampitt) disclose a method of treating a subterranean formation which includes the use of a fluid medium comprising an aqueous gel having a water-soluble cellulose ether, a water-soluble compound of a polyvalent metal wherein the metal is capable of being reduced to a lower polyvalent valence state and a reducing agent. A variety of cellulose ethers are set forth as being suitable.
U.S. Pat. No. 3,888,312 (Tiner et al.) describes a fracturing fluid including an aqueous liquid, a gelling agent and a cross-linking compound, which fluid has a viscosity while in laminar flow of 25 centipoises and greater up to about 100,000 centipoises, but which, while in turbulent flow, such as in a conduit, exhibits a resistance to fluid flow of less than that of water. The gelling agents useful are various solvatable polysaccharides having a molecular weight of at least about 100,000 including galactomannan gums and cellulose derivatives, and hydroxyethylcellulose derivatives having between 0.5 and about 10 moles of ethylene oxide per anhydroglucose unit. A suitable crosslinking compound includes the presence of titanium in the +4 oxidation state. U.S. Pat. No. 4,033,415 (Holtmyer et al.) discloses a similar aqueous gel for fracturing and placing propping agents within a subterranean formation, and numerous cross-linking agents are disclosed.
U.S. Pat. No. 4,144,179 (Chatterji) discloses a composition for treating low temperature subterranean formations comprising a gelled aqueous composition which includes an aqueous liquid, a water soluble organic gelling agent, a free radical generating agent, and a reducing agent. Water soluble organic gelling agents disclosed include water soluble derivatives of cellulose, including hydroxyethyl cellulose, carboxymethylhydroxyethylcellulose, carboxymethylcellulose, and the like.
U.S. Pat. No. 4,323,123 (Swanson) describes gelled compositions suitable as fracture fluids and the like which comprise water, a polymeric viscosifier, an aldehyde component and at least one phenolic component such as resorcinol. Any of the water-soluble cellulose ethers may be used to prepare the aqueous gels.
U.S. Defensive Publication No. T103,401 (Majewicz) discloses that, if carboxymethyl cellulose is modified with hydroxyethyl functionality, the salt sensitivity of the aqueous solutions thereof is improved to the point where use as a thickener for fracturing fluids is provided. Carboxymethylhydroxyethylcellulose having a carboxymethyl degree of substitution of 0.7 to 1 and a hydroxyethyl molecular substitution of 0.3 to 2 can be so employed. Cross-linking of the cellulose derivatives employed may be effected by means of trivalent and tetravalent metal ions, including aluminum, titanium and chromium.
Despite this considerable effort, state-of-the-art fracturing fluids utilize, as the water soluble polymer, guar gum or guar gum derivatives, such as hydroxypropyl guar gum. Guar gum has the capability of being cross-linked by a variety of polyvalent metals, including chromium, titanium, boron and zirconium. In comparison to other polymer systems utilized, it is generally considered that "gelled" fracturing fluids utilizing guar gum and the like develop not only adequate viscosity to provide support for proppants but, while not entirely satisfactory, at least provide improved performance upon being exposed to high rates of shear and elevated temperatures in comparison to such other polymer systems. However, and unfortunately, guar and guar derivatives contain substantial amounts of insoluble matter, typically amounting from about 5% up to about 10% or more by weight; and it has been postulated that degradation may provide additional quantities of insoluble matter. It is widely accepted that the presence of such insoluble matter results in formation damage, viz.--a reduction in the productivity that would otherwise be obtained. The supply and availability of guar gum also involves some uncertainty.
Cellulose ethers such as hydroxyethyl cellulose would on the surface appear to be excellent alternatives to guar and guar derivatives for use in fracturing fluids. Thus, hydroxyethyl cellulose should be capable of being degraded without any undue residue that would cause formation damage. It is also believed that hydroxyethyl cellulose itself should inherently possess greater thermal stability, or may be stabilized by additives to provide such greater thermal stability, than guar gum and the like.
Despite this theoretical potential hydroxyethyl cellulose or its derivatives have not been utilized in fracturing fluids to any substantial extent, at least in the more demanding applications. Prior attempts using hydroxyethyl cellulose have thus been unable to provide fracturing fluids with shear-thermal stability characteristics which in service provide characteristics considered comparable to those achieved through the use of guar gum and the like. Hydroxyethyl cellulose and its derivatives have heretofore been unable to be adequately crosslinked to provide the characteristics generally desired for useful fracturing fluids, particularly in high temperature applications. Accordingly, despite the accepted view that the use of guar gum and the like results in formation damage, such materials continue to be the materials of choice for use in hydraulic fracturing fluids, especially in the more demanding applications.
U.S. Pat. No. 4,001,210 (Engelskirchen et al.) discloses a process for manufacturing cellulose containing 2,3-dihydroxypropyl ether groups. Neutralization with a source of borate ions such as boric acid provides substantially increased viscosity of such cellulose derivatives in an aqueous solution in comparison to the use of, for example, hydrochloric acid. The etherification reaction may be carried out with glycidol or a derivative which reacts like glycidol under the reaction conditions. Suitable examples of these derivatives include its easily saponifiable derivatives, such as the lower alkanoic acid esters of glycidol, glycidol acetate being exemplified. Suitable suspending agents for the reaction include secondary lower alkanols such as isopropanol, tertiary alcohols, ketones such as 2-butanone, and cyclic oxaalkanes and dioxaalkanes.
U.S. Pat. No. 4,013,821 (Engelskirchen et al.) has as an object to provide a process for preparing cellulose ethers which can be easily washed out in the presence of borate ions and a solution of which is distinguished by an especially high viscosity. Cellulose ethers which contain one or several alkyl, hydroxyalkyl, or carboxyalkyl substituents and a total degree of substitution of 0.05-4.0 are converted into the corresponding 2,3-dihydroxypropyl mixed ethers by reaction with glycidol and/or a glycerol monohalohydrin such as 1-halo-2,3-dihydroxypropane. The use of glycidol is preferred since the glycerol monohalohydrin contains dihalo compounds that are only very difficultly separable and result in cross-linking reactions taking place.
U.S. Pat. No. 4,096,326 (Reid) discloses the reaction product of 3-chloro-1,2-propanediol or glycidol and cellulose ethers to provide ethers that are water-soluble. The resulting product forms useful complexes with certain borate and antimonate compounds which are highly stable in brine. Desired products are achieved over a fairly wide range of the dihydroxypropyl substituent levels, ranging from about 1.4 up to 6 dihydroxypropyl groups per anhydroglucose unit.
Yet, despite all of this prior technology and effort, there remains the need for a water-soluble polymer capable of use in hydraulic fracturing fluids and the like which will provide in service shear-thermal stability characteristics at least substantially equivalent to those achieved through use of guar gum and the like without the attendant potential for formation damage.